The present invention concerns the field of solid state capacitors and relates particularly to massed production methods for manufacturing solid state capacitors.
A massed production method for solid state tantalum capacitors is described in U.S. patent specification No. 5,357,399 (inventor Ian Salisbury). This method involves providing a substrate wafer of solid tantalum, forming a sintered, highly porous, layer of tantalum on the substrate, sawing the layer of porous tantalum with an orthogonal pattern of channels to produce an array of upstanding porous tantalum rectilinear bodies, anodising the cubes to produce a dielectric layer on the bodies, dipping the bodies in manganese nitrate solution and heating to convert the applied solution to manganese dioxide thereby to form a cathode layer, applying respective conducting layers of carbon and then silver onto top ends of each body, bonding a lid consisting of a wafer of solid metal onto the silver layer; injecting insulating resin material into the channels between bodies constrained by the substrate and lid; and slicing the assembly in a direction perpendicular to the plane of the wafers and along the centre line of each channel thereby to produce a plurality of capacitors in which the anode terminal consists of substrate material, the cathode terminal consists of lid material and the capacitive body consists of the coated porous tantalum body.
This method allows the production of highly compact, reliable capacitors of high volumetric efficiency. However with the continued miniaturization of components demanded by the electronics industry there is pressure to produce even smaller and more efficient capacitors.
The present invention seeks to provide a novel method for the massed manufacture of solid state capacitors which allows further improvements in volumetric efficiency and/or further miniaturization of capacitors.
According one aspect of the present invention there is provided a method of manufacturing multiple solid state capacitors comprising:
providing a metal substrate layer;
forming on an upper surface of the substrate layer a plurality of upstanding bodies consisting of porous sintered valve-action metal;
forming a dielectric layer on the bodies;
forming a cathode layer on the dielectric layer;
coating a top end of each upstanding body with at least one conducting intermediary layer by liquid or vapour phase deposition or by application of an immobilized flowable composition such as a solidifiable paste, thereby to form an intimate physical contact between the cathode layer and the intermediate layer;
encapsulating side walls of each body with an electrically insulating material; and
dividing the processed substrate into a plurality of individual capacitor bodies each having a sleeve of encapsulating material, an anode terminal surface portion at one end consisting of exposed substrate and a cathode terminal surface portion at the other end consisting of exposed intermediary layer.
By using an intermediary layer as an exposed cathode terminal surface portion it is possible to omit a solid lid layer from the capacitor. This allows a considerable improvement in the volumetric efficiency of the capacitor formed because space previously taken up by the lid in the prior art method can be allocated to the porous valve-action metal.
According to another aspect of the invention the encapsulation process comprises juxtaposing a solid lid on the respective top ends of the anode bodies, introducing the encapsulation material in a liquid phase to occupy any free space between the lid layer and substrate, causing or allowing the encapsulating material to solidify and removing the lid from the top ends, whereby the sidewalls of each upstanding body are encapsulated without contamination of the juxtaposed portions of the top ends of the bodies.
The encapsulation may involve a preliminary stage in which e.g. powdered thermoplastics resin is introduced into the spaces between the upstanding bodies and then melted by heating of the substrate to form a layer of thermoplastic partway up the sides of each body. Preferably this preliminary part-encapsulation is conducted using a resin of different coloration to the main encapsulation resin, thereby providing a visible polarity indication in the final capacitors. Alternatively polarity may be indicated by other marking such as laser etching.
According to yet another aspect of the invention an intermediary layer is coated onto the bodies by applying intermediary layer material onto a surface of the or a lid, followed by juxtaposition of the lid on the anode bodies so that the top ends are contact-coated by transfer of the material from the lid to the respective top ends, and thereafter removing the lid.
A release agent is preferably provided between the lid and body top ends, which agent facilitates removal of the lid after encapsulation. Preferably the release agent comprises a high surface energy polymer layer formed on the lid. One suitable polymer is PTFE. Where contact coating is part of the process, the layer to be coated is applied onto the release agent which is itself applied to the lid.
The intermediary layer material may be applied to the body top ends by screen printing of an immobilized paste of layer material onto the lid.
Preferably pressure is applied to the lid in order to ensure an intimate contact between the lid and the upstanding body ends. In addition where a coating has been applied to the lid, the pressure ensures effective transfer of material from the lid to the body ends.
The intermediary layer may be formed by solidification of a conducting paint or paste. The layers may be applied by dipping into paste solutions.
In a preferred embodiment two intermediary layers are applied by dipping, and a final intermediary layer is applied by contact coating onto the second layer.
In one embodiment, one intermediary layer comprising carbon is coated on the cathode layer and a further intermediary layer comprising silver is coated on the carbon layer.
The capacitor bodies may be formed into useful capacitors by a termination process in which the respective exposed cathode and anode surfaces of each capacitor body are liquid or vapour phase coated with a termination material which facilitates electrical connection of the respective ends of the capacitor to an electric circuit.
The respective terminal coatings may form a cap on each end of the capacitor body, as in the industry-standard five-sided termination processes.
In a preferred embodiments the valve-action metal is tantalum. However other valve-action metals may be used in the process of the present invention. Examples are niobium, molybdenum, silicon, aluminium, titanium, tungsten, zirconium and alloys thereof. Preferred examples are niobium and tantalum.
When the valve action metal is tantalum the substrate is preferably a solid tantalum wafer, thereby ensuring physical and chemical compatibility with the porous metal.
The upstanding anode bodies may be formed by a process which involves pressing a layer of valve-action metal powder onto the substrate and sintering to fuse the powdered particles. Typically a seeding layer of coarse grade powder may have to be applied to the substrate and sintered thereto before finer grade powder is pressed onto the substrate. The coarse grade powder provides mechanical keying ensuring that a strong connection between the substantive porous layer and the substrate is produced. The strong connection is necessary to ensure that separation of the porous layer from the substrate does not occur during subsequent steps in the manufacturing process. The coherent layer of porous valve action metal thereby produced may be machined or otherwise processed to produce the individual anode bodies. The bodies may be formed by machining of a porous sintered layer formed on the substrate. The machining may be by means of orthogonal sawing to form rectilinear bodies.
According to a further aspect of the invention there is provided a capacitor produced by any method hereinbefore described.
According to another aspect of the invention there is provided an electronic or electrical device comprising a capacitor made by any method hereinbefore described.
The dielectric layer may be formed by an electrolytic anodization process in which an oxide film is carefully built up on the surface of the porous sintered anode body. Suitable methods will be known to the person skilled in the art.
The cathode layer may be formed by dipping the anode bodies into a cathode layer precursor solution such as manganese nitrate and then heating to produce a cathode layer of manganese dioxide. Repeated dipping and heating steps may be carried out in order gradually to build up the required depth and integrity of cathode layer.
Typically, during the dipping process the cathode layer will be built up not only on the anode bodies, but also on the exposed tantalum substrate surface between bodies. In order that each cathode-terminal is isolated from its respective anode terminal a further process step may be carried out to remove any cathode layer (and dielectric layer) from the substrate around the anode body. This process may involve a further machining process in which isolation channels are formed between each anode body by removal of a surface layer of substrate. For example, where orthogonal rows have been machined to form rectilinear anode bodies, isolating channels may be machined along the centre lines of the rows and columns between anode bodies. In this way, a step is formed in the perimeter of each capacitors anode body, which step has an uncoated surface, thereby isolating the cathode layer from the exposed anode terminal.
With the application of the cathode layer, the anode body becomes a capacitive body comprising an anode portion consisting of an interconnected matrix of: metal powder; dielectric insulating layer of metal oxide; and a conducting cathode layer of doped oxide.
The encapsulating resin may be applied under pressure or by simple immersion depending upon the suitability and fluidity of the particular resin. Once the resin has set, the resin and substrate may be machined or otherwise cut to separate adjacent capacitor bodies. The encapsulation material may be a plastics resin, such as epoxy.